ActiveAdminで各ページ共通でレイアウトの構成要素をいじる ref: http://qiita.com/skuroki@github/items/c5fce4b45a2d1a4a4bca

base.rb

module ActiveAdmin
  module Views
    module Pages
      class Base
        def build_main_content_wrapper
          div id: "main_content_wrapper" do
            div id: "main_content" do
              para <<-'EOS'.html_safe
<a href="https://twitter.com/share" class="twitter-share-button">Tweet</a>
<script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0],p=/^http:/.test(d.location)?'http':'https';if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src=p+'://platform.twitter.com/widgets.js';fjs.parentNode.insertBefore(js,fjs);}}(document, 'script', 'twitter-wjs');</script>
              EOS
              main_content
            end
          end
        end
      end
    end
  end
end